1. Field of the Invention
The present invention relates to a method of defect inspection of a graytone mask and an apparatus doing the same.
2. Description of the Related Art
In recent years, attempts have been made to cut down the number of mask sheets by using graytone masks in the field of large-sized LCD masks (as set forth in the monthly FPD Intelligence, May, 1999).
As shown in FIG. 4A, such a graytone mask has a opaque part 1, a transmission part 2 and a graytone part 3 on a transparent substrate. The graytone part 3 corresponds to an area in which a opaque pattern 3a of not exceeding the resolution limit of an exposure apparatus for a large-sized LCD using the graytone mask is formed, for example, and is designed to selectively change the thickness of a photoresist film by decreasing the light transmitted through this area so as to decrease the amount of irradiation due to the area, 3b showing a microscopic transmission part of not exceeding the resolution limit of the exposure apparatus in the graytone part 3. Normally, the opaque part 1 and the opaque pattern 3a are formed with films that are made of the same material such as chromium (Cr) or a chromium compound and have the same thickness. The transmission part 2 and the microscopic transmission part 3b are transparent substrate parts, each without having a opaque film on the transparent substrate. The resolution limit of the exposure apparatus for the large-sized LCD using the graytone mask is about 3 μm in the case of an exposure apparatus of a stepper type and about 4 μm in the case of an exposure apparatus of a mirror projection type. Consequently, the space width of a transmission part 3b in the graytone part 3 of FIG. 4A is set at less than 3 μm and the line width of the opaque pattern 3a of not exceeding the resolution limit of the exposure apparatus is set at less than 3 μm, for example. When the exposure apparatus for the large-sized LCD is used for light exposure, as the exposure light transmitted through the graytone part 3 as a whole is deficient in the amount of light exposure, positive photoresists are left on a substrate though the thickness of the positive photoresists exposed to light via the graytone part 3 solely decreases. More specifically, there arises a difference in solubility of resists in developing liquid between parts corresponding to the ordinary opaque part 1 and to the graytone part because of difference in the amount of light exposure and this results in, as shown in FIG. 4B, making apart 1′ corresponding to the ordinary opaque part 1 as thick as about 1.3 μm, making a part 3′ corresponding to the graytone part 3 as thick as about 0.3 μm and making a part corresponding to the transmission part 2 a part 2′ without resists, for example. A first etching of a substrate as a workpiece is carried out in the part 2′ without the resists so as to remove the resists in the thin part 3′ corresponding to the graytone part 3 by ashing and the like and by carrying out a second etching of this part, the etching process is performed with one sheet of mask instead of two sheets of masks as conventionally used in order to cut down the number of masks for use.
A conventional method of inspection of a mask having only opaque and transmission parts will now be described.
FIG. 7A shows a condition in which a clear defect 4 (pinhole) and a opaque defect 5 (spot) are produced in a opaque part 1 and a transmission part 2 respectively with both parts being scanned by one of the lenses (hereinafter called an upper lens) of a comparative inspection apparatus as shown by an arrow.
FIG. 7B shows an amount-of-transmission signal 7 obtainable along the scanning line of the lens. The amount-of-transmission signal 7 is detected by a CCD line sensor disposed in each lens unit, for example. The level of the amount-of-transmission signal 7 is B in the opaque part 1 and W in the transmission part 2. The transmittance of the opaque part 1 is set at 0% and the transmittance of the transmission part 2 is set at 100%. The amount-of-transmission signal 7 is basically formed of a pattern edge line signal (pattern form signal) generated at the edge (boundary between the opaque part and the transmission part) of the pattern. In case where defects are produced, there appear a clear defect signal 4′ generated in the opaque part 1 and a opaque defect signal 5′ generated in the transmission part 2.
FIG. 7C shows an amount-of-transmission signal 7′ obtainable by the other lens (hereinafter called a lower lens) in case where no defect is produced even in the same pattern as that of FIG. 7A.
FIG. 7D shows a difference signal 8 obtained by subtracting the amount-of-transmission signal (a different portion) of each lens; more specifically, there is shown therein a difference signal obtained by subtracting the amount-of-transmission signal 7′ of FIG. 7C from the amount-of-transmission signal 7 of FIG. 7B. In the difference signal 8, only defect signals 4′ and 5′ are extracted because a pattern edge line signal is removed from the amount-of-transmission signal of each lens.
FIG. 7E shows a condition in which with the setting of thresholds necessary for extracting defects in the opaque part 1 and the transmission part 2 in the difference signal 8 that has extracted only defect signals, the clear defect is detected by a plus-side threshold 9a and the opaque defect is detected by a minus-side threshold 9b. Although the detection sensitivity increases as the thresholds lower, the thresholds are needed to be set at a level on which no false defects are picked up.
In order to make sure that what kind of defect is produced in which one of the lenses, the signal of the upper lens is compared with that of the lower lens in a upper lens circuit (by subtracting the signal of the lower lens from that of the upper lens), for example, so as to detect clear and opaque defects in the upper lens because a defect signal appears on the plus side when the clear defect is produced in the opaque part 1 of the upper lens and because a defect signal appears on the minus side when the opaque defect is produced in the transmission part 2 of the upper lens (FIG. 7B-(5)). In the same way, the signal of the lower lens is compared with that of the upper lens in a lower lens circuit (by subtracting the signal of the upper lens from that of the lower lens), for example, so as to detect clear and opaque defects in the lower lens because a defect signal appears on the plus side when the clear defect is produced in the opaque part 1 of the lower lens and because a defect signal appears on the minus side when the opaque defect is produced in the transmission part 2 of the lower lens.
As the conventional comparative inspection apparatus mentioned above is a apparatus for inspecting a conventional mask only having a opaque and a transmission part, it is unable to inspect a graytone mask having a graytone part.
More specifically, in case where thresholds are set as those necessary for extracting defects in the opaque and transmission parts as stated above, the defect signal in the graytone part is weak since a pattern forming the graytone part is microscopic and since the defect itself is normally very small, so that the thresholds are too high to extract the defect in the graytone part.
On the assumption that the thresholds are set as those necessary for extracting the defect in the graytone part, the defects in the opaque and transmission parts are not extractable and moreover these false defects are not distinguishable from the defect of the graytone part since false defects in the opaque and transmission parts are picked up, so that no defect in the graytone part can be inspected.
Further, only one line of the defect extracting threshold is allowed to be set on the plus side and on the minus side.